The present invention is directed, in general, to a copper-based electroplating solution and, more specifically, to a copper-based electroplating solution for electroplating copper onto a semiconductor wafer to form an copper layer on which an under bond metal may be formed.
The number of levels within ultra large-scale integration circuits (ULSIs) over the last decade has increased tremendously. Current ULSIs are approaching six levels. This increase in levels has required interconnect technologies to adjust accordingly. A major limiting factor in interconnect technology is RC delay, introduced by the coupling characteristics of metals and insulators. An efficient interconnect scheme, for advanced ULSIs, requires materials with low effective time constants. In this regard, metals with low resistivity, such as copper and other noble metals, are emerging as materials of choice. However, even though other materials may be used, copper is currently the interconnect material of choice.
Unfortunately, however, there are integration problems associated with the use of copper in forming interconnects within the integrated circuit. One such problem is the difficulty in packaging the device wafers. In order to make copper metalization manufacturable, the ability to package the finished wafers is of utmost importance. Current wiring techniques, such as gold wire-bonding and flip chip technologies, may not work on copper metalized wafers. Thus, there is a need for under bond metalization (UBM).
Critical to UBM technology are the mechanical properties of the outermost copper layer, to which the UBM will be attached. Specifically, the adhesion at the UBM/Cu and Cu/dielectric interface must be robust. These interfaces must survive thermomechanical cycling, which deforms the interconnect. Thus, the outermost copper layer must be engineered to afford high yield strengths as well as good adhesion to the underlying dielectric and the UBM.
Critical to the yield strength and adhesion strength of the outermost copper layer are the processing conditions for the copper. Such processing conditions include the electroplating bath composition, chemistry of the surface and texture of the seed layer. Currently, the semiconductor manufacturing industry uses the same electrodeposition solution for all levels within the integrated circuit. However, the current electrodeposition solutions do not provide adequate mechanical yield strength to the electroplated copper that is used to support the under bond metal. Thus, when the current electroplated copper interconnect is subjected to thermomechanical cycling, the yield strength and adhesion strength may decrease to an unacceptable level.
Accordingly, what is needed in the art is an electrodeposition solution that may be used in current copper interconnect technology and that does not experience the mechanical yield strength problems associated with the prior art solution.
To address the above-discussed deficiencies of the prior art, the present invention provides an aqueous electroplating solution. In a preferred embodiment, the aqueous electroplating solution includes a copper salt, an acid that has a dissociation constant of less than about 2, a hydrogenated halide and a modulator that comprises a weight by weight percent of the electroplating solution that is less than about 1%. In a preferred embodiment, the copper salt comprises a weight by weight percent of the electroplating solution that ranges from about 0.1% to about 2.5%. The acid comprises a weight by weight percent of the electroplating solution that ranges from about 0.1% to about 10%, and the hydrogenated halide and the modulator each comprise a weight by weight percent of the electroplating solution that ranges from about 0.0001% to about 1%.
Thus, in one aspect, the present invention provides an aqueous electroplating solution that yields a superior electroplating solution that can be used to form an improved interconnect structure. This unique solution provides an electroplated interconnect structure with beneficial yield and adhesion strength values such that the electroplated interconnect structure can provide a support structure for an under bond metal (UBM).
In one embodiment, the copper salt is copper sulfate, which preferably comprises a weight by weight percent of the electroplating solution of about 0.3%. In another embodiment, the acid is sulfuric acid, which in a preferred embodiment, comprises a weight by weight percent of the electroplating solution of about 0.08%. The acid, however, in other embodiments may be any of a number of strong acids, including inorganic acids or organic acids that have disassociation constants about equal to or less than sulfuric acid. In yet another embodiment, the hydrogenated halide may be a hydrogen bromide, hydrogen chloride, or hydrogen iodine. The hydrogenated halide preferably comprises a weight by weight percent of the electroplating solution of about 0.001%, respectively.
In another embodiment, the modulator comprises a brightener. The brightener preferably includes an acidic aqueous solution of formaldehyde, urea, beta-diketonates, thiourea, diamino-urea, or thiophene. In one particular embodiment, the brightener is a derivative of 2,2xe2x80x2-thiobisalkyl acetoacetate or 2,2xe2x80x2-aminobisalkyl having a general formula: (Xxe2x80x94[CH2(CH2)nxe2x80x94Oxe2x80x94C(O) CH2C(O)CH3]2), where X=S or N respectively. When X=S and n=2, in another embodiment, the 2,2xe2x80x2-thiobisalkyl acetoacetate is 2,2xe2x80x2-thiobisethyl acetoacetonate. Furthermore, when X=N and n=2, in another embodiment, the 2,2xe2x80x2-aminobisalkyl acetoacetate is 2,2xe2x80x2aminobisethyl acetoacetonate. However, in a preferred embodiment, the brightener includes an acidic aqueous solution comprising alkylthiol, having a general formula: (Rxe2x80x94SH), where R=CH3(CH2)nxe2x80x94CH2 when nxe2x89xa72.
The modulator, in an alternative aspect, may function as a leveler. Furthermore, the modulator may be a mixture of the above discussed brightener, and the leveler and may further comprise a weight by weight percent of the electroplating solution less than about 0.01%. Additionally, the leveler may include the brightener and a water soluble polymer. The water soluble polymer may be a polyalcohol, or more specifically saturated and unsaturated aliphatic polyalcohols, e.g. a polyalkylene glycol.
Another aspect of the present invention provides a method of fabricating an integrated circuit on a semiconductor wafer. The method includes (1) forming transistors, such as metal oxide semiconductor (CMOS) transistors or bipolar transistors, on a semiconductor wafer, (2) forming a dielectric layer over the transistors, (3) forming an opening in the dielectric layer, (4) placing the semiconductor wafer into the aqueous electroplating solution described above, (5) electroplating copper in the opening, and (6) interconnecting the transistors to form an operative integrated circuit.
One embodiment further includes, depositing a barrier layer within the opening. In yet another embodiment, the method further includes depositing a copper seed layer on the barrier layer. The barrier layer may be tantalum/tantalum nitride, titanium/titanium nitride, tantalum/tantalum silicon nitride, or titanium/titanium silicon nitride.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.